T. W. Hagen

Observation of Large Scissors Resonance Strength in Actinides

The orbital M1 scissors resonance has been measured for the first time in the quasicontinuum of actinides. Particle-γ coincidences are recorded with deuteron and (3)He-induced reactions on (232)Th. The residual nuclei (231,232,233)Th and (232,233) Pa show an unexpectedly strong integrated strength of B(M1)=11-15μ(n)(2) in the E(γ)=1.0-3.5 MeV region. The increased γ-decay probability in actinides due to scissors resonance is important for cross-section calculations for future fuel cycles of fast nuclear reactors and may also have an impact on stellar nucleosynthesis.

Constant-temperature level densities in the quasicontinuum of Th and U isotopes

Particle-gamma coincidences have been measured to obtain gamma-ray spectra as a function of excitation energy for 231-233Th and 237-239U. The level densities, which were extracted using the Oslo method, show a constant temperature behavior. The isotopes display very similar temperatures in the quasi-continuum, however, the even-odd isotopes reveal a constant entropy increase Delta S compared to their even-even neighbors. The entropy excess depends on available orbitals for the last unpaired valence neutron of the heated nuclear system. Also, experimental microcanonical temperature and heat capacity have been extracted. Several poles in the heat capacity curve support the idea that an almost continuous melting of Cooper pairs is responsible for the constant-temperature behavior.

Transitional

The level densities and γ-ray strength functions of 105 ,106,111,112Cd have been extracted from particle-γ coincidence data using the Oslo method. The level densities are in very good agreement with known levels at low excitation energy. The γ-ray strength functions display no strong enhancement for low γ energies. However, more low-energy strength is apparent for 105,106Cd than for 111,112Cd. For γ energies above ≈4 MeV, there is evidence for some extra strength, similar to what has been previously observed for the Sn isotopes. The origin of this extra strength is unclear; it might be due to E1 and M1 transitions originating from neutron skin oscillations or the spin-flip resonance, respectively.

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We investigated the U238(d,p) reaction as a surrogate for the n+U238 reaction. For this purpose we measured for the first time the γ-decay and fission probabilities of U∗239 simultaneously and compared them to the corresponding neutron-induced data. We present the details of the procedure to infer the decay probabilities, as well as a thorough uncertainty analysis, including parameter correlations. Calculations based on the continuum-discretized coupled-channels method and the distorted-wave Born approximation (DWBA) were used to correct our data from detected protons originating from elastic and inelastic deuteron breakup. In the region where fission and γ emission compete, the corrected fission probability is in agreement with neutron-induced data, whereas the γ-decay probability is much higher than the neutron-induced data. We have performed calculations of the decay probabilities with the statistical model and of the average angular momentum populated in the U238(d,p) reaction with the DWBA to interpret these results.

strength in Cd isotopes

Author(s): Renstrom, T; Nyhus, HT; Utsunomiya, H; Schwengner, R; Goriely, S; Larsen, AC; Filipescu, DM; Gheorghe, I; Bernstein, LA; Bleuel, DL; Glodariu, T; Gorgen, A; Guttormsen, M; Hagen, TW; Kheswa, BV; Lui, YW; Negi, D; Ruud, IE; Shima, T; Siem, S; Takahisa, K; Tesileanu, O; Tornyi, TG; Tveten, GM; Wiedeking, M | Abstract:

Investigation of the U238(d,p) surrogate reaction via the simultaneous measurement of γ-decay and fission probabilities

Author(s): Renstrom, T; Nyhus, HT; Utsunomiya, H; Schwengner, R; Goriely, S; Larsen, AC; Filipescu, DM; Gheorghe, I; Bernstein, LA; Bleuel, DL; Glodariu, T; Gorgen, A; Guttormsen, M; Hagen, TW; Kheswa, BV; Lui, YW; Negi, D; Ruud, IE; Shima, T; Siem, S; Takahisa, K; Tesileanu, O; Tornyi, TG; Tveten, GM; Wiedeking, M | Abstract:

Low-energy enhancement in theγ-ray strength functions ofGe73,74

CITATION: Kheswa, B. V., et al. 2017. ¹³⁷,¹³⁸,¹³⁹La(n,γ) cross sections constrained with statistical decay properties of ¹³⁸,¹³⁹,¹⁴⁰La nuclei. Physical Review C, 95(4):1-9, doi:10.1103/PhysRevC.95.045805.

Low-energy enhancement in the γ-ray strength functions of 73,74Ge

Abstract.It is almost 80 years since Hans Bethe described the level density as a non-interacting gas of protons and neutrons. In all these years, experimental data were interpreted within this picture of a fermionic gas. However, the renewed interest of measuring level density using various techniques calls for a revision of this description. In particular, the wealth of nuclear level densities measured with the Oslo method favors the constant-temperature level density over the Fermi-gas picture. From the basis of experimental data, we demonstrate that nuclei exhibit a constant-temperature level density behavior for all mass regions and at least up to the neutron threshold.

La137,138,139(n,γ) cross sections constrained with statistical decay properties of La138,139,140 nuclei

Nuclear level densities and γ-ray strength functions of 56,57Fe have been extracted from proton-γ coincidences. A low-energy enhancement in the γ-ray strength functions up to a factor of 30 over common theoretical E1 models is confirmed. Angular distributions of the low-energy enhancement in 57Fe indicate its dipole nature, in agreement with findings for 56Fe. The high statistics and the excellent energy resolution of the large-volume LaBr3(Ce) detectors allowed for a thorough analysis of γ strength as function of excitation energy. Taking into account the presence of strong Porter–Thomas fluctuations, there is no indication of any significant excitation energy dependence in the γ-ray strength function, in support of the generalized Brink–Axel hypothesis.

Experimental level densities of atomic nuclei

The nuclear level densities of